Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing, or support layer; one or more unexposed photocurable layers; a protective layer or slip film; and often a protective cover sheet.
The support sheet or backing layer lends support to the plate. The support sheet, or backing layer, can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred materials include steel, copper, or aluminum sheets, plates, or foils; paper; or films or sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like. The support sheet can optionally comprise an adhesive layer for more secure attachment to the photocurable layer(s). Optionally, an adhesion layer or an antihalation layer may also be provided between the support layer and the one or more photocurable layers. An antihalation layer is used to minimize halation caused by the scattering of UV light within the non-image areas of the photocurable resin layer.
The photocurable layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material. Preferred photocurable materials include an elastomeric compound, an ethylenically unsaturated compound having at least one terminal ethylene group, and a photoinitiator. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzrnacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. If a second photocurable layer is used, i.e., an overcoat layer, it typically is disposed upon the first layer and is similar in composition.
The photocurable materials generally cross-link (cure) and harden through radical polymerization in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of effecting a chemical change in an exposed moiety. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions. One suitable source of actinic radiation is a mercury arc lamp, although other sources are generally known to those skilled in the art.
The slip film is a thin layer, which protects the photopolymer from dust and increases its ease of handling. In a conventional plate making process, the slip film is transparent to UV light. In this process, the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate. Instead of a slip film, a matte layer may also be used to improve the ease of plate handling. The matte layer typically comprises fine particles (silica or similar) suspended in an aqueous binder solution. The matte layer is coated onto the photopolymer layer and then allowed to air dry. A negative is then placed on the matte layer for subsequent UV-flood exposure of the photocurable layer.
In a “digital” or “direct to plate” plate making process, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative on a digital (i.e., laser ablatable) masking layer, which is generally a slip film which has been modified to include a radiation opaque material. Portions of the laser ablatable layer are ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. Examples of laser ablatable layers are disclosed for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety.
After imaging, the photosensitive printing element is developed to remove the unpolymerized portions of the layer of photocurable material to create a relief image on the surface of the photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or heat plus a blotter. The resulting surface has a relief pattern that reproduces the image to be printed. The printing element may then be mounted on a press and printing commenced.
Photocurable resin compositions typically cure through radical polymerization, upon exposure to actinic radiation. However, the curing reaction can be inhibited by molecular oxygen, which is typically dissolved in the resin compositions, because the oxygen functions as a radical scavenger. It is therefore preferable that the dissolved oxygen be removed from the resin composition before image-wise exposure so that the photocurable resin composition can be more rapidly and uniformly cured.
The removal of the dissolved oxygen may be accomplished by placing the photosensitive resin plate in an atmosphere of inert gas, such as carbon dioxide gas or nitrogen gas, overnight before exposure in order to displace the dissolved oxygen. The drawback to this method is that it is inconvenient and cumbersome and requires a large space for the apparatus. This approach has generally been replaced by subjecting the plates to a preliminary exposure (i.e., “bump exposure”).
During bump exposure, a low intensity “pre-exposure” dose of actinic radiation is used to sensitize the resin before the plate is subjected to the higher intensity main exposure dose of actinic radiation. The bump exposure is applied to the entire plate area and is a short, low dose exposure of the plate that ostensibly reduces the concentration of oxygen, which inhibits photopolymerization of the plate (or other printing element). Without this pre-sensitization step, fine features (i.e., highlight dots, fine lines, isolated dots, etc.) are not preserved on the finished plate. However, the pre-sensitization step often tends to cause shadow tones to fill in, thereby reducing the tonal range of the halftones in the image. This is exacerbated in plate formulations that have very high sensitivity and small exposure latitude.
The bump exposure also requires specific conditions that are limited to only quench the dissolved oxygen, such as exposing time, irradiated light density and the like. In addition, if the photosensitive resin layer has a thickness of more than 0.1 mm, the weak light of the low intensity bump exposure dose does not sufficiently reach certain portions of the photosensitive resin layer (i.e., the side of the photosensitive layer that is closest to the substrate layer and furthest from the source of actinic radiation), at which the removal of the dissolved oxygen is insufficient. In the subsequent main exposure, these portions will not cure sufficiently due to the remaining oxygen. Accordingly, after developing, the photosensitive layer may contain isolated dots having barrel-like shapes and thinning toward the substrate layer, thus resulting in the decline of durability.
If an attempt is made to quench the dissolved oxygen sufficiently in the side of the photosensitive layer that is closest to the support layer during the preliminary exposing step, the surface of the resin layer which is closest to the actinic radiation source receives too much radiation and undergoes unwanted cure. Also, if the main exposure is conducted for a longer period of time in order to sufficiently cure the side of the of the resin layer which is closest to the support layer, the etching depth of reverse portions may be shallow and positive features may end up rendered bolder than the original film negative. Thus, there remains a need in the art for improved methods of bump exposing relief image printing plates to overcome the drawbacks of the prior art.
Bump exposures are generally carried out with fluorescent UV tubes or medium pressure mercury arc lamps. However, the spectral output of these lamps is extremely wide, and their output spans wavelengths from below 320 nm in the UV region to over 800 nm in the infrared region, a span of more than 480 nm.
With such a broadband source, it is not possible to maximize the depth of penetration of the bump exposure radiation while maintaining good exposure speed. Some wavelengths may penetrate well, while other wavelengths may be absorbed strongly at the surface. The net result is that the maximum bump dose the plate can withstand and still process leaves the resin at the bottom of the photosensitive layer (i.e., the surface of the resin layer closest to the support sheet) significantly under-bumped and still rich with inhibiting oxygen.
Bump exposure dosages are normally determined empirically, by first increasing the exposure time until evidence of polymerization is seen, and then backing off by 5–10 percent. Because the photopolymer layers near the surface receive the highest dose of actinic radiation, what happens chemically in these layers dictates when the photopolymer as a whole has received its maximum bump exposure. Thus, it can be seen that the top layers of resin are essentially depleted of oxygen while the lower layers may still contain significant amounts of oxygen. This gradient of oxygen content throughout the resin thickness can lead to undercutting of some image features and reduced overall exposure sensitivity.
Other efforts to improve the removal of dissolved oxygen from relief image printing plates have involved special plate formulations alone or in combination with the bump exposure.
For example, U.S. Pat. No. 5,330,882 to Kawaguchi, the subject matter of which is herein incorporated by reference in its entirety, suggests the use of a separate dye that is added to the resin to absorb actinic radiation at wavelengths at least 100 nm removed from the wavelengths absorbed by the main photoinitiator. This allows separate optimization of the initiator amounts for the bump and main initiators. Unfortunately, these dyes are weak initiators and require protracted bump exposure times. In addition, these dyes sensitize the resin to regular room light, so inconvenient yellow safety light is required in the work environment. Lastly, the approach described by Kawaguchi employs conventional broadband-type sources of actinic radiation light for bump exposure, and thereby also tends to leave significant amounts of oxygen in the lower layers of the resin.
U.S. Pat. No. 4,540,649 to Sakurai, incorporated herein by reference in its entirety, describes a photopolymerizable composition that contains at least one water soluble polymer, a photopolymerization initiator and a condensation reaction product of N-methylol acrylamide, N-methylol methacrylamide, N-alkyloxymethyl acrylamide or N-alkyloxymethyl methacrylamide and a melamine derivative. According to the inventors, the composition eliminates the need for pre-exposure conditioning and produces a chemically and thermally stable plate.
U.S. Pat. No. 5,645,974 to Ohta et al., incorporated herein by reference in its entirety, discloses a photocurable mixture that includes paraffin or a similar waxy substance to inhibit effect of atmospheric oxygen. Due to its low solubility in the polymer, the paraffin floats at the beginning of the polymerization and forms a transparent surface layer that prevents the ingress of air. This approach does nothing to eliminate molecular oxygen already dissolved in the resin composition.
Other efforts have focused on adding an oxygen scavenger to the resin composition to suppress the action of the oxygen. The use of oxygen scavengers in resin systems is described, for example, in U.S. Pat. No. 3,479,185 to Chambers, Jr. and in U.S. Pat. No. 4,414,312 to Goff et al., the subject matter of each or which is herein incorporated by reference in its entirety.
Although various methods of inhibiting and/or removing dissolved oxygen in the photosensitive resin composition have previously been suggested, there remains a need in the art for an improved method of removing dissolved oxygen from the entirety of the photosensitive resin layer.
The inventors have surprisingly discovered that if the bump exposure is carried out by using substantially monochromatic actinic radiation (i.e., the range of wavelengths of the source of the radiation differ by no more than about 20 nm) and matching its wavelength with a photoinitiator of the correct type and concentration such that the optical density of that photoinitiator in the resin at the emitted wavelength is less than about 0.43, the bump exposure more effectively and uniformly removes the molecular dissolved oxygen throughout the entire resin thickness.